Machining of spur and helical gears by processes utilizing grinding wheels of the type commonly referred to as globoidal, hyperboloidal or hourglass, have been known in the art for some time.
A process for producing crowned gear teeth by linearly moving a work gear relative to the concave edge of a grinding wheel having a helical groove is disclosed by U.S. Pat. No. 3,110,135 to Berlinsky. In this process, the work gear must be moved at an angle across the edge of the grinding wheel in order to produce straight or convex tooth forms having crowned shapes. The angular movement requires a special mechanism on the grinding machine which inherently complicates the machine and process. Moving the work gear straight across the edge of the grinding wheel produces an unusable concave tooth form and no crowning.
German Patent Application 25 16 059 describes a plunge process wherein a hyperboloidal or globoidal grinding worm is utilized for grinding spur and helical gears. The grinding wheel axis and work gear axis are crossed, that is, the projections of the axes into a plane that is parallel to both axes intersect at an angle of less than ninety degrees. This type of axes orientation enables the effective area of the tool to extend from one side face of the work gear to the other so that only a radial change in the distance between the axes is needed for machining. In other words, additional movement of the work gear along its axis to ensure complete machining of the tooth length is not required and the only motion needed is the plunge feed of the grinding wheel relative to the work gear. However, due to stock allowances on the work gear teeth, the tool and work gear surfaces will engage long before the operational center distance is reached. Some portions of the grinding wheel thread will engage the work gear for a longer period of the feed sequence than others and also remove more material. These portions of the grinding wheel will wear faster and cause profile shape problems. Also in this type of "full thread" process, grinding forces would be very high due to the amount of contact between the grinding wheel and the work gear. The process itself is not capable of grinding some gears. The tooth surfaces of the grinding wheel would bind with the work gear before the operational center distance is reached thereby causing operating portions of the gear profiles to be removed.
The method described by Fragin et al. in "New Method of Finishing Teeth of Hardened Spur Gears", (Vestnik Mashinostroeniva, Volume 55, Issue 7, 1975), sets forth a plunge-type honing process. One flank of each of the work gear teeth is machined by meshing a revolving globoidal abrasive worm with the one flank, the abrasive thread being narrower than the finished tooth gap. The necessary honing force is created by braking the work gear. Simultaneous with the rotation of the grinding worm and work gear, the work gear is given a reciprocating motion along its axis in order that the machining is complete along the entire tooth length. The opposite flank of each of the work gear teeth is machined by reversing the rotation of the grinding worm. Accurate control of the braking force and reciprocating motion is complex and, therefore, constant production of an acceptable gear is difficult.
Another plunge-type process is disclosed by U.S. Pat. No. 4,559,744 to Wirz. In this process a grinding worm is provided with a thread thickness smaller than the finished tooth gap dimension of the work gear. The grinding worm and work gear are rotated at a revolution ratio corresponding to the number of teeth of the grinding worm and work gear. The grinding worm is then fed relative to the work gear until a desired distance is reached between the axes of the grinding worm and work gear with no contact occurring. At this point the threads of the grinding worm are located between adjacent tooth flanks of the work gear teeth. The grinding worm or work gear is then given an additional rotating movement which is superimposed on the basic rotation thereby enabling a first flank of each of the work gear teeth to be machined. The other flank of each of the teeth is machined by providing an additional rotating movement in a direction opposite that provided for machining the first flank.
A process similar to that of U.S. Pat. No. 4,559,744 is described in U.S. Pat. No. 4,650,378 to Zubler. In this process, after the tool and work gear are radially moved to the desired center distance, at least one of the tool and work gear are moved in a direction perpendicular to the direction of radial movement in order to machine a first flank of each of the work gear teeth. The other flank of each of the work gear teeth is machined by moving in a direction opposite to the direction required for machining the first flank.
The latter two processes, like that of Fragin et al. discussed above, are very complex in that control of the additional rotational motion, or, motion in the plane perpendicular to the radial direction, is difficult to accurately implement and monitor. Also, it is known in plunge-type processes that the fewer the number of teeth on a work gear, the smaller the face width capable of being machined. Furthermore, only one size work gear can be machined with each particular grinding wheel tooth form. A change in size of work gear necessitates that the grinding wheel be dressed with a dressing tool having the same size as the desired work gear.
It is an object of the present invention to provide a method of and machine for machining spur and helical gears wherein the deficiencies of the prior art are eliminated and a wider range of gear sizes are capable of being machined.
It is another object of the present invention to provide a method and machine wherein a plurality of gear sizes can be machined with a single grinding tool.
It is yet another object of the present invention to provide an improved method of dressing a grinding tool having a generally hourglass shape along the width thereof.